Glycosphingolipids and gangliosides are of crucial importance as major membrane constituents of the cell, the majority of which are located at the outer leaflet of the plasma membrane. Recent investigations have demonstrated that glycosphingolipids on cell surfaces are one important way in which nature expresses its individuality.
Sphingosine is one of the more important 2-amino 1,5 diols. Sphingosine (or sphingenine) is a ubiquitous membrane component of the natural glycolipids: cerebrosides, sphingomyelins and gangliosides as described in Sphingolipid Biochemistry, Plenum Press, N.Y., N.Y.(1983)by J. N. Kanfer and S. Hakomori. This long chain aminoalcohol and its metabolites play important roles in a variety of biological events. ##STR1##
As the lipophilic fragment of glycolipids, sphingosine and its fatty acid derivative ceramide act as an anchor in the phospholipid bilayer. The nature of the lipid and its interaction with the phospholipid bilayer directly influence the manner in which the hydrophilic carbohydrate moiety is presented to the extracellular environment. On the other hand, the stereo specificity of membrane organization as well as the rigidity of the bilayer depends upon the composition of and interaction between the various lipid components.
Glycosphingolipids have numerous extracellular roles. For example, they serve as specific markers of the cells, particularly those forming blood group antigens, tumor cell markers, cell adhesion organ specific markers and growth regulators. They have been also implicated as receptors for toxins, hormones and interferons.
A prominent intracellular regulatory function of sphingosine and its derivatives has been shown. It has also been shown that sphingosine and lysosphingolipids, as components of complex membrane lipids, serve as natural endogenous inhibitors of protein kinase C. The latter has been shown to play a central role in signal transduction for those receptors that function via the phospholipid-dependent second messenger system, modulating cell growth and differentiation.
More recently, dermatological studies on mice have shown that sphingosine inhibits all of the protein kinase C-mediated responses including inflammation, hyperplasia and induction of ornithine decarboxylase activity suggesting a therapeutic application in inflammatory skin diseases.
The complete biological and pharmacological function of sphingosine still remains to be elucidated. Therefore, the development of an efficient stereoselective route to this material and its diastereomers on a multigram scale is still an important synthetic goal.
The important structural features of natural D-(+)-erythro-sphingosine include the absolute configuration at the two chiral centers of this aminodiol (2S, 3R) and the trans geometry of the double bond. Various syntheses of racemic sphingosine have appeared in the literature following the pioneering total synthesis described by Shapiro, et al in "The Total Synthesis of Sphingosine", J. Am.
Chem. Soc., Vol. 80, pages 1194, 2170 (1958).
Over the past decade, a number of asymmetric syntheses of sphingosine were published using various synthetic strategies and chiral starting materials. The oldest and perhaps easiest preparation of crude D-sphingosine involves the hydrolysis of the natural sphingolipids. However, the main drawbacks of this procedure are the great expense of the starting materials and the tedious purification of the sphingosine (sphingenine) from dihydrosphingosine (sphinganine) and other impurities. These same drawbacks also apply to chiral resolution techniques using racemic sphingosine.
The presently known synthetic methods for preparation of chiral D-erythro-sphingosine can be ultimately divided into three major categories:
1. Methods utilizing Wittig olefination for preparation of the trans-double bond from configurationally labile 4-carbon .alpha.-hydroxyaldehydes. Several chiral .alpha.-substituted butyraldehyde derivatives were used in this method which were usually obtained from chiral carbohydrate precursors: D-mannose, D-glucose, D-galactose and D-xylose. PA0 2. Methods which employ optically active .alpha.-amino-.alpha.,62 -ynone chemistry, preparation of which is based on the 3-carbon backbone of serine and utilizing the (2S)-stereochemistry of L-serine as a chiral precursor to build up the chiral center at carbon 3 described by D. Liotta et al in "A Stereoselective Synthesis of Sphingosine, A Protein Kinase C Inhibitor", Tetrahedron Letters, Vol. 29, No. 25, pages 3037-3040 (1988). PA0 3. There are various other miscellaneous procedures described in the art. Of the miscellaneous procedures which are not unreasonably complex, there are two methods used to introduce the olefin: PA0 The former method represents the most attractive procedure for introduction of the trans-olefinic bond as exemplified by the salt-mediated method of Schmidt and co-workers in "Synthesis of D-Erythro-sphingosines", Tetrahedron Lett., Vol. 27, page 481 (1986). However, we and other investigators have been unable to reproduce the reported yield and high stereoselection to give "practically exclusively" the trans-alkene product. See "Synthesis of D-Erythro-1-Deoxydihydroceramide-1-Sulfonic Acid" by K. Ohashi, et al, Tetrahedron Letters, Vol 29, page 1185 (1988). As a result, several groups have pursued a photochemical cis/trans isomerization with further separation of the isomers, or the Schlosser modification of the Wittig olefination reaction as disclosed in the following reference: M. Schlosser and K. F. Christmann, Ann. Fur Chemie, Vol. 708, Page 1(1967).
(a) Direct stereoselective Wittig olefination. PA1 (b) Wittig olefination employing the Schlosser modification.
One of the three most efficient methods of synthesizing sphingosine to date is described by D. Liotta, et al in "A Stereoselective Synthesis of Sphingosine" supra, hereinafter referred to as the "Liotta" reference. The Liotta process involves eight (8) steps starting from the chiral precursor, L-serine, and has an overall yield of approximately 27%. The disadvantages of this process are the great expense of time and money and inefficiency involved in carrying out an eight step process as compared to a process with a lesser number of steps and the inclusion of some racemic yield.
The second of the three most efficient methods of synthesizing sphingosine is described by H. Radunz, et al in "An Efficient and Stereoselective Synthesis of D-erythrosphingosine", supra, hereinafter referred to as the "Radunz" reference. The "Radunz" reference describes a process almost identical to the one described in the Liotta reference. However, it involves seven (7) steps starting from L-serine and has an overall yield of approximately 12-14%.
The third of the three most efficient methods of synthesizing sphingosine is described by P. Zimmermann and R. R. Schmidt in "Synthesis of Erythro-sphingosines Via Their Azido Derivatives", Liebigs Ann, Chem., pages 663-667 (1988). This method begins with chiral carbohydrate precursors, namely D-galactose(or D-xylose). This procedure involves a total of six (6) steps. The overall yield is only 16%.
Further, the reduction of the azide to amine requires the use of hydrogen sulfide which is known to be inherently dangerous.
Therefore, prior to this invention, there has been a need for a process for the preparation of chiral 2-amino-1,3-diols which combines the advantages of less expense, exclusively chiral product, good overall yield, and safety of reagents.
To further understand the nature of this invention, a reaction scheme is provided hereinbelow: ##STR2##
The oxazolidinone aldol adduct formed in Step A is more fu11y described by Alexander L. Weiss and Carl L. Illig in commonly-owned U.S. Ser. No. 428,800 filed on even date herewith and entitled "Oxazolidinone Aldol Adduct".